Port Orleans French Quarter is such a charming resort, Monica. If you're looking for a home in Disney's moderate resort category, it's certainly a good pick. With children as young as yours--and plans to spend the majority of your time at Magic Kingdom--a resort on the monorail line would be a more convenient choice. It would also, however, come at a premium, so if a moderate is more in line with your budget then you should feel confident in your choice to stay at French Quarter.
I would encourage you to branch out a bit from the Magic Kingdom. Each of the four major theme parks offer something for young children, though, admittedly, Magic Kingdom is a favorite for preschoolers. At Hollywood Studios, kids can boogie with their friends from
Disney Junior - Live on Stage! At Epcot,
Turtle Talk with Crush is a big hit with the younger crowd. And at Animal Kingdom,
The Boneyard provides lots of room to run off some of that energy! Even if you spent just a half day at each of the other three parks, you'd have the chance to get a feel for all that Walt Disney World has to offer.
With that said, if you plan to attend only the Magic Kingdom, a four or five day trip would be plenty long enough. It would give you the opportunity to spend three days at Magic Kingdom, a day lounging by your resort pool, and a little time to explore Downtown Disney, too. Enjoy your vacation!